As for the solid-state imaging device, there is widely used a flat light-receiving device where photoelectric conversion sites are two-dimensionally arrayed in a semiconductor to form pixels and a signal generated by photoelectric conversion in each pixel is charge-transferred and read out through a CCD or CMOS circuit. The conventional photoelectric conversion site generally used is a photodiode part formed using PN junction in a semiconductor such as Si.
In recent years, fabrication of a multipixel device is proceeding, and the pixel size and in turn, the area of a photodiode part become small, which causes problems of reduction in the aperture ratio, reduction in the light collection efficiency and the resulting reduction in the sensitivity. As for the measure to increase the aperture ratio and the like, studies are being made on a solid-state imaging device having an organic photoelectric conversion film using an organic material.
A technique of introducing a bulk heterojunction structure using a fullerene or a fullerene derivative into the organic photoelectric conversion film so as to bring out high photoelectric conversion efficiency (high exciton dissociation efficiency) is known. For example, Patent Document 1 discloses a photoelectric conversion film containing a fullerene or a fullerene derivative.
The organic photoelectric conversion element used in a solar cell has a purpose of collecting electric power and therefore, an external electric field is not applied, but the photoelectric conversion element used as a visible light sensor of a solid-state imaging device needs to maximize the photoelectric conversion efficiency and a voltage is sometimes externally applied so as to improve the photoelectric conversion efficiency or enhance the response speed.
When a voltage is externally applied so as to improve the photoelectric conversion efficiency or enhance the response speed, injection of a hole or an electron from an electrode is generated due to an external electric field, and this disadvantageously increases the dark current.
Many of materials usually used as an electrode in a photoelectric conversion element have a work function (WF) of around 4.5 eV (for example, ITO) and, for example, in the case of using a fullerene (C60) as the material of the photoelectric conversion film, an energy gap between WF of the electrode and LUMO of the fullerene (C60) becomes small, as a result, particularly an electron is liable to be injected from the electrode into the photoelectric conversion film and a significant increase of dark current is caused.
As regards the prevention of an increase in the dark current due to an injected current, a technique of providing a charge blocking layer to suppress the injection of an electric charge into the photoelectric conversion layer, thereby efficiently blocking an injected carrier and reducing the dark current, is disclosed (Patent Document 2).
In Patent Documents 1 and 2, heat resistance becoming an important factor in practice is not referred to, and a chemical structure having high heat resistance is not sufficiently described.
Patent Documents 3 to 6 disclose an organic material having a hole transporting property, such as fluorene and carbazole, but are silent on a photoelectric conversion element and also lacking sufficient description regarding dark current and heat resistance.
Patent Document 7 discloses an organic material having a fluorene skeleton, but this material is used in a dye-sensitized solar cell and since the characteristics required of a solar cell differ from those required of a photoelectric conversion element aiming at an imaging device component, unlike the present invention, description regarding dark current and heat resistance is insufficient.
Also, in the case of producing a film by using the compound described in Patent Document 7, generation of a grain boundary by crystallization due to low amorphous property and formation of unevenness on the film surface may occur, and this material is not suited as a material of a photoelectric conversion element aiming at a photosensor, an imaging device or the like.